In recent years, there has been proposed a technique of directly communicating user data (data of User-Plane) among a plurality of radio terminals without passing through a radio base station (D2D communication). The communication of user data directly performed among the plurality of radio terminals is performed by using a part of radio resources assigned to a mobile communication system. However, in the D2D communication, the communication of control data (C-Plane) is performed via the radio base station, similarly to a conventional mobile communication system.